Electrochemical gas sensors, such as oxygen sensors, are normally designed to operate in a diffusion limited mode. This is normally achieved by using a capillary or membrane, which limits gas access in a well defined and repeatable way. The sensor is designed such that the capillary or membrane provides the limiting factor. For example, the gas sensing electrode is designed to have sufficient activity reserve that the actual activity of the electrode can generally be ignored (since it is much greater than required to consume the available gas). Under certain conditions, however, sensors can deviate from the ideal diffusion limited behaviour: for example, if the catalytic activity of the electrode falls significantly, or if blocking or flooding of the membrane supporting the electrode occurs, then the sensor current may fall below the ideal diffusion limited current. In other cases, faults can occur with the diffusion limiting component, such as cracking or damage to the housing, resulting in a higher diffusion limited current, or conversely partial blocking of the capillary or diffusion limiting membrane can result in a lower diffusion limited current.
In addition to such faults which can result in a change in the level of gas response, certain faults can result in a reduction in the speed of response, even though the steady state response remains unchanged: for example, if the electrode-supporting membrane becomes partially flooded this causes initially a slow response, then in extreme cases a reduction in steady state response. The same is true if the catalytic activity of the electrode is reduced, which can be a particular problem for sensors with low activity reserve.
It is desirable to be able to detect and diagnose such faults, preferably with the sensor in a measuring instrument (i.e. in situ) and without user intervention. It is also desirable to be able to extract information about sensor parameters such as the catalytic activity, and the presence and correct operation of internal components, as an end of line production test for example. If such faults or changes in sensor performance can be reliably and simply detected, the resulting information could be used to indicate when maintenance is due, or to modify data processing algorithms to compensate for changes in performance, for example.
Some of the above mentioned faults or parameters can be detected by “gas testing” the sensor. Examples of such techniques are disclosed in EP-A-0260005, U.S. Pat. No. 6,165,347 and U.S. Pat. No. 5,741,413. In each case, a sensor to be calibrated is exposed to a known amount of gas and the resulting current produced by the sensor is analysed and used to calibrate the output, based on the known volume and/or concentration of gas. In EP-A-0260005 and U.S. Pat. No. 6,165,347, the test is conducted by filling a chamber of known volume with the test gas and exposing the sensor to the chamber. In U.S. Pat. No. 5,741,413 the tiny “dead” volume within the sensor itself is used as the test gas chamber, which is sealed (or has its communication with the external atmosphere much reduced) by the use of a valve mechanism which at least partially closes capillary access into the sensor.
However, gas testing the sensor in this way is often not feasible in the field, since access to sensors in situ may be difficult or even dangerous. In addition, such techniques require additional mechanical components, increasing the cost, size and complexity of the instrument. Further, the nature of the gas test is such that the sensor must be taken out of normal operation while it is exposed to the test gas, and remain so while the necessary measurements are taken. Moreover, such gas tests can only detect faults which are severe enough to take the sensor out of its normal diffusion limited operating regime. Likewise, U.S. Pat. No. 5,558,752 describes a method of determining whether a sensor signal is limited by diffusion or kinetics by determining whether the sensor current varies with the bias potential applied to the sensor.
U.S. Pat. No. 6,428,684 discloses a diagnostic technique in which the sensing circuit is momentarily broken while the flux of gas continues into the sensor. The short transient signal generated when the sensor is switched back on is compared with the steady state current and the time for which the sensor circuit was kept open, in order to determine whether the amount of gas consumed during the transient is equal to that which would be expected to be consumed had the reaction continued at its steady state level for the open circuit duration. Thus, the test is simply able to determine whether the sensor as a whole is operating under diffusion control or not. However, there are many scenarios in which, despite a fault, the sensor continues to operate under diffusion control and this test will not be able to identify such problems.
It is desirable to be able to measure such parameters which are normally masked by the diffusion limiting behaviour of the sensor. For example, in a sensor which is gradually losing catalytic activity, a gas test such as those described above will only detect this when the activity has become the limiting factor, which in many situations is too late. It is desirable to obtain an early warning of impending failure, for example by determining that the activity has fallen below a safe level (that level still being above the level needed for the sensor to be diffusion limited) or to monitor the change in activity over a time, to predict the remaining lifetime. Similarly, as previously mentioned, in new sensors, it is desirable to have a catalyst activity significantly higher than necessary in order to provide excess activity reserve to allow for loss of activity over the sensor lifetime and with effects such as varying temperature. It is desirable to be able to perform a simple end of line production test to check that the activity is within a certain range (again, while the sensor is diffusion limited). Conventional gas tests at ambient temperature on the assembled sensor will not allow this to be measured.